An inkjet recording apparatus used as an image recording apparatus (an image forming apparatus) such as a printer, a facsimile, a copier, a plotter and the like is provided with an inkjet head as a droplet discharging head. The inkjet head includes nozzles for discharging ink droplets, ink channels (the ink channel may be called a discharge chamber, a pressure chamber, a pressurizing fluid chamber, a fluid chamber, a pressurizing chamber or the like) each of which is connected to the nozzles, and pressure generation parts for pressurizing ink in the ink channels. Although there are various kinds of droplet discharging heads such as for discharging fluid resists as droplets, or for discharging a sample of DNA as droplets, for example, the inkjet head will be mainly described as in the following description.
As for the inkjet head, a piezo type (Japanese laid-open patent application No. 2-51734), a thermal type (Japanese laid-open patent application No. 61-59911), and an electrostatic type (Japanese laid-open patent application No. 6-71882) are known. In the piezo type, a vibration plate forming a wall of the ink channel is deformed by using a piezoelectric element that is a pressure generation part for pressurizing ink in the ink channel, so that the volume of the ink channel is changed and ink droplets are discharged. In the thermal type, ink droplets are discharged by using pressure caused by bubbles that are generated by heating ink in the ink channel by using a heating resistor. In the electrostatic type, the vibration plate that forms a wall of the ink channel and an electrode are placed opposingly, and the vibration plate is deformed by using electrostatic force between the vibration plate and the electrode, so that the volume of the ink channel is changed and ink droplets are discharged.
In these inkjet heads, either of two methods is used for discharging ink droplets. One method is a “push and shoot” method in which the vibration plate is pushed toward the pressurizing chamber, so that the volume of the pressurizing chamber is decreased and ink droplets are discharged. Another method is a “pull and shoot” method in which the vibration plate is deformed by a force toward the outside of the ink chamber first (away from the nozzles)), and then the vibration plate is returned to its original position, such that the volume that is once enlarged is returned to its original the volume, so that ink droplets are discharged.
For example, a domestic re-publication of PCT international publication for patent application No. WO95/10416 discloses a driving method of the piezo type head using the “pull and shoot” method. The PCT application discloses a driving method used for the inkjet head for discharging ink in the pressurizing chamber by using a stacked piezoelectric actuator unit, wherein the stacked piezoelectric actuator unit includes a substrate and a plurality of rows each including a pair of stacked piezoelectric actuators. The stacked piezoelectric actuator has piezoelectric distortion constant d33 and is provided with collection electrodes on both end surfaces, and the pair of stacked piezoelectric actuators are arranged on the substrate such that the pair of stacked piezoelectric actuators are opposed to each other. In the driving method, in first step, a voltage is applied to the stacked piezoelectric actuators in a polarization direction of the stacked piezoelectric actuators so as to lengthen the stacked piezoelectric actuators in the thickness direction. In second step, ink is filled in the pressurizing chamber by decreasing the voltage gradually. In third step, the ink is discharged by lengthening the stacked piezoelectric actuators in the thickness direction by abruptly increasing the voltage again.
However, in the conventional “pull and shoot” method by using the above-mentioned piezoelectric element (piezoelectric vibrator) of d33 deformation, there is a problem in that the voltage is always applied to the piezoelectric element even when printing is not performed, so that reliability of the piezoelectric element, and by extension, reliability of the head, decreases.
As another example of the inkjet head adopting the “pull and shoot” method, Japanese laid-open patent application No.11-268266 discloses an inkjet printer that adopts the “pull and shoot” method. Japanese laid-open patent application No. 11-268266 discloses a driving signal for a piezoelectric vibrator in an inkjet head, in which the driving signal includes pulses for controlling the head in the following way.
A potential difference ΔV1 of the driving signal between before and after expansion of a pressure chamber is set to be greater than a potential difference ΔV2 of the driving signal between before and after contraction of the pressure chamber. Accordingly, the pressure chamber is contracted from a state in which the meniscus (free surface) of ink is largely pulled from the nozzle aperture, so that an ink droplet for a small dot is discharged. The weight of the ink droplet can be further decreased by optimizing the driving signal for the small dot, so that the diameter of the recorded dot can be further decreased.
However, there is a problem in that it is difficult to optimize the driving signal for the small dot only by setting the potential difference ΔV1 of the driving signal to be greater than the potential difference ΔV2 of the driving signal.
That is, according to verification by the inventor of the present invention, it is necessary to perform optimization between a discharge pulse (discharge pulse 114 in Japanese laid-open patent application No.11-268266, “discharge pulse” means “electrical discharge pulse” hereinafter) included in the driving signal and a charge pulse (charge pulse 116 in Japanese laid-open patent application No. 11-268266) in order to make the most of pressure vibration in the ink pressure chamber that occurs when applying the discharge pulse. That is, it is necessary to optimize the voltage holding time during which constant voltage is kept, time for applying the discharge pulse, and time for applying the charge pulse. That is, it can be realized to set the potential difference ΔV1 to be greater than the potential difference ΔV2 only when such optimization is realized.
As still another example of a conventional technology, Japanese laid-open patent application No.6-297707 discloses an inkjet recording apparatus, in which the volume of a pressure chamber is expanded and ink is filled in the pressure chamber, and, after that, ink is discharged by contracting the volume of the pressure chamber. In this process, the speed for expanding the volume of the pressure chamber in the first stage is changed according to recording characteristics of a recording medium, so that only ink discharge amount can be freely changed while ink discharge speed is kept to be constant.
As for an inkjet head that uses high viscosity ink, it is necessary to shorten the time for refilling ink from an ink supply chamber for obtaining good frequency characteristics. Therefore, fluid resistance Ro of a fluid resistance part that connects the ink pressure chamber and the ink supply chamber needs to be small. In a case where the inkjet head is driven by a conventional driving signal having a pulse waveform shown in FIG. 1 for an inkjet recording apparatus adopting the “pull and shoot” method, when volume expanding speed of the ink pressure chamber is large (that is, when ΔV/Tfs shown in FIG. 1 is large), a negative pressure in the ink pressure chamber becomes large, and the supply of ink from the ink supply chamber is performed speedily since the fluid resistance Ro is small. Therefore, pulled depth of the nozzle meniscus cannot be large. That is, as shown in FIG. 1, a discharge pulse 101 is output for a period of time Tfs during which the voltage decreases from the voltage of the holding pulse 100. Then, a holding pulse 102 (voltage Vpb) is output for a time period of Pws, and a charge pulse 103 in which the voltage increases for a time period Trm is output. After that, the voltage of the pulse becomes Vps (holding pulse 104). On the other hand, if the volume expanding speed of the ink pressure chamber is decreased, pressure in the ink supply chamber cannot be increased. Thus, it cannot be expected to realize efficient ink discharge by using the pressure in the ink supply chamber.
FIG. 2 shows a relationship between the time Pws in the pulse waveform and the depth of the meniscus from the nozzle surface of the inkjet head. In FIG. 2, the voltage Vps is applied to the piezoelectric vibrator by the holding pulse 100, so that the piezoelectric vibrator is charged and extended. As a result, volume of the ink pressure chamber decreases. Next, the piezoelectric vibrator is extended by discharging the piezoelectric vibrator to the voltage Vpb by the discharge pulse 101, so that the volume of the ink pressure chamber is expanded. At this time, pressure occurs in the ink supply chamber, wherein the magnitude of the pressure vibrates at a period Ts. Thus, since negative pressure occurs first, the meniscus is pulled toward the inside of the ink pressure chamber. Then, ink starts to be supplied gradually from the ink supply chamber. As a result, as the ink is supplied, the meniscus that is once pulled in gradually rises to the surface of the nozzle while the meniscus performs damped vibration for a period of Ts. Considering that high viscosity ink is used and fluid resistance Ro is small, when voltage ΔV is set to be constant and the time Tfs is set to be short, the meniscus depth is small and the amplitude of the vibration is large. If the time Tfs is set to be longer, the meniscus depth becomes deep and the amplitude becomes small. It is known that the meniscus depth has a close relationship with ink droplet amount to be discharged, and the amplitude of the vibration has a close relationship with ink discharge speed. That is, when it is intended to obtain a small ink droplet by using a large meniscus depth, desired ink discharge speed cannot be obtained. Thus, a large discharge voltage is necessary. However, when ink discharge speed is increased by using a large discharge voltage, the ink discharge amount becomes large at the same time. Thus, the desired size of a small ink droplet cannot be obtained.
As for the technique disclosed in Japanese laid-open patent application No.6-297707, volume expanding speed of the pressure chamber of the inkjet head can be changed freely, so that only the ink discharge amount can be changed freely. However, the ink discharge speed becomes slow. Therefore, printing speed is lowered, and printing image quality is lowered due to variations of positions of ink droplets projected on a recording medium.